Fuller langchain integration for streaming support

[AriMKatz]

I believe I read somewhere that contingent on interest, there are plans for more extensive lcel integration.

Please consider this interest:)

Steaming is critical to our use case and other production niceties would be great too. We can't really use dspy without that.

Edit: this is the quote:

If we convert this into a fuller integration, all users stand to benefit. LangChain users will gain the ability to optimize any chain with any DSPy optimizer. DSPy users will gain the ability to export any DSPy program into an LCEL that supports streaming and tracing, and other rich production-targeted features in LangChain.